In the recent progress of optical communication technology, LAN (local area network) wiring, inter-equipment wiring, etc., for optical communication have come to be employed. To an optical cable used in optical LAN wiring, inter-equipment optical wiring, etc., for indoor or in-vehicle use, high flexibility and thin diameter are desired in consideration of use in special environments with congested portions and uneven portions.
An optical cable used in such a special environment is comparatively short in length (1 m to 20 m) in comparison to a normal optical cable (optical cable for long-haul communication). Such a short-length optical cable is thus not required to be as low in transmission loss as an ordinary optical cable. In contrast, when an optical cable is used in a special environment, a flame retardant is contained at a weight percentage of 30% or more in a cable jacket in many cases. This is because when an optical cable is small in external dimensions, it contacts air readily due to being large in surface area per unit volume and because a halogen-free flame retardant of low flame retardancy needs to be used in consideration of environmental problems.
Also, optical LAN wiring, inter-equipment wiring, etc., are used, not for long-haul optical communication but for short-haul optical communication between equipments installed in a special environment, such as indoors, inside a vehicle, etc. Thus in an optical cable premised on use in a special environment, a multimode optical fiber of large core diameter, which is advantageous in optical linking property as compared with single mode optical fibers, is applied in many cases. However, as compared with such a general-purpose single mode optical fiber, a multimode optical fiber is susceptible to disturbance factors, such as thermal expansion and contraction, etc., and increases in transmission loss readily. A soft, ultraviolet curing acrylate resin (soft layer) with a Young's modulus of approximately 1 MPa is thus used as a coating layer for a coated optical fiber so that the disturbance factors can be relaxed.
As optical cables with a comparatively low number of coated optical fibers and small external cable dimensions, there are optical cables of shapes known as drop cables and indoor cables (see, for example, Patent Document 1). Normally, in this type of optical cable, tension members are disposed at opposite sides of one to several coated optical fibers, and the coated optical fibers and the tension members are integrally covered by a cable jacket formed of a thermoplastic resin. Also, in Patent Document 1, by positioning a fibrous interposed member at a periphery of the coated optical fibers, the optical cable is provided with impact resistance and lateral pressure resistance characteristics.
Furthermore, in an optical cable used in a special environment, such as indoors, in a vehicle, etc., a high bending performance is required for use as wiring in a congested location or wiring in an opening/closing portion of a door. Also, connection to equipment is generally achieved using an optical connector, and because there are cases where optical connectors are attached onsite and to reduce manufacturing costs even in cases where manufacturers attach optical connectors to optical cables, optical cables that are excellent in optical connector attachment property are being demanded.
As a conventional drop optical cable or indoor optical cable, there is known a cable structure, such as shown in the area (a) of FIG. 14, where tension members 3, each constituted by a steel wire of approximately 0.4 mm diameter are disposed at opposite sides of coated optical fibers 2, and the coated optical fibers 2 and the tension members 3 are integrally covered with a cable jacket 4 in a manner such that a cable outer diameter becomes approximately 2 to 4 mm (see, for example, Patent Document 2). As the tension member 3, there is also known a stranded steel wire, formed by stranding a plurality of thin steel wires, as shown in the area (b) of FIG. 14. Furthermore, as a structure of the tension member 3, there is known a structure, with which, in place of steel wires, glass fibers or aramid fibers, etc., are made integral by binding with resin.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-144821    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-198588